In recent years digital microphones has becoming increasingly popular in portable electronic equipment and, in particular, as used with mobile phones. One advantage of digital microphones is their inherent property of being very immune to modulated RF signals, both radiated and conducted.
For example, microphones are typically placed in close vicinity to radio transmitters, i.e., the antenna, in many mobile phones. Previously, analog microphones have been used in mobile phones, but these are quite susceptible to modulated RF signals such as noise coming from the antenna. In an analog microphone the modulated RF signal is demodulated into an unwanted audio signal.
Digital microphones do not face many of the same demodulation issues or concerns as analog microphones. For instance, the immunity of digital microphones towards modulated RF signals opens the possibility of placed in close proximity to the antenna. However, this displacement creates new problems.
More specifically, the antenna of a typical mobile phone is not only used to transmit RF signals but also used to receive RF signals. The received RF signals are often very small, e.g., approximately −140 dBm, and thus are very sensitive to interfering signals.
As the output signal from the digital microphone is digital, then the output signal will have very steep edges (e.g., nS) and thus the frequency content of the signal reaches into several hundreds of MHz (and sometimes into the GHz range). This creates interference problems for the circuit.
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